Welcome to Axiom. Yes, slightly change the dimensions to avoid the 7:21 factor. Your seating position would establish the best speaker placement. The M80s should be separated about the same amount as your distance from them. The QS8 side surrounds should be mounted directly to the side of the listening position and about 2-3' above ear level. Back surrounds would be helpful if there's enough distance(say at least 4')behind the listening position for a rear sound field to form. If so, mount them on the back wall, also about 2-3' above ear level and separated by about the distance they're behind the seating.
